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Assessment of the Folin and Ciocalteu's Method for Determining Soil Phenolic Carbon

Dep. of Applied Ecology and Environmental Sciences, Univ. of Maine, 5722 Deering Hall, Orono, ME 04469-5722.

^* Corresponding author (ohno{at}maine.edu).

ABSTRACT

Previous studies have shown that phenolic compounds are potentially allelopathic and present at low concentrations in soil solution. This study was conducted to evaluate the Folin and Ciocalteu's method for determining concentrations of phenolic C in aqueous and citrate extracts of soils containing 7.1 to 57.9 g kg^–1 organic C. Soils were reacted for 18 h with a 0.3 mM phenolic acid mixture or DI-H₂O to serve as a control. Folin and Ciocalteu's reactive-phenolic C content in the DI-H₂O control extracts suggested positive interference from dissolved organic matter rather than free phenolic compounds desorbed from the native soils. The absorptivity of the Folin and Ciocalteu's reduced complex was 1128 ± 25 L mol^–1 cm^–1 for the phenolic acid mixture and 45 ± 8 L mol^–1 cm^–1 for soluble organic matter ligands. Although the method has much higher sensitivity to phenolic acids than to organic matter, the higher concentration of organic matter results in substantial background positive interference. However, multiple regression equations of corrected Folin and Ciocalteu's absorbance (phenolic-treated absorbance minus deionized water control absorbance) regressed with solution parameters had excellent fits (R² > 0.97). This suggests that interference correction is possible in aqueous extracts. The Folin and Ciocalteu's reactive-phenolic C concentration in the citrate extracts of the phenolic acid mixture-amended soils were not significantly different from the DI-H₂O control soil extracts indicating that the Folin and Ciocalteu's method is not suitable for use with soil citrate extractions. The large quantity of organic matter extracted by citrate probably masks absorbance due to any extracted phenolic compounds.

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